The purpose of the proposed project is to determine the mechanisms by which amino acid transport is regulated in 4-cell and blastocyst staged mouse embryos and in trophoblastic cells derived from blastocysts. Detailed kinetic studies will be done by incubating the cells in radioactive amino acids for specific times and then analyzing acid-soluble and acid-insoluble fractions for transport and incorporation into protein, respectively. Kinetic studies will also be done on the effects upon transport of 1) competing unlabeled amino acids, 2) inhibitors of energy metabolism and macro--molecular syntheses, 3) cell membrane active agents, 4) depletion of Na ion and other ions. These experiments should elucidate the nature and mechanism of specificity for amino acid transport, the sources of energy for amino acid transport, and the role and mechanism of action of Na ion in amino acid transport in developing mammalian embryos. The Na ion studies, in which comparisons are made between different developmental stages and different amino acids already present an excellent opportunity to study the regulation of developmental changes in cell membrane function. The information obtained from the proposed work will be useful in selecting and designing abortifacients which are specifically effective upon preimplantation embryos and/or trophoblasts cells. If such agents are found for trophoblastic cells, which are invasive, it would be appropriate to test their effects upon cancer cells.